1. Field of the Invention
This invention relates generally to a vibrating compressor, and more specifically to a vibrating compressor wherein a counter electromotive force generated in a drive coil vibrating in a magnetic field is detected to switch an external drive current in synchronism with said vibration of said drive coil, and said external drive current is adapted to be supplied to said drive coil so as to efficiently facilitate the vibration of said drive coil.
2. Description of the Prior Art
Refrigerators using vibrating compressors have heretofore been widely used on board recreational vehicles. When a refrigerator of this type is equipped on board a recreational vehicle, said vibrating compressor is operated by the d-c power source of the vehicle by converting the voltage of the d-c power source (battery) into an alternating voltage in a d-c/a-c converter circuit to apply to the drive coil of said compressor. Since the frequency of the alternating voltage as an output of said d-c/a-c converter circuit is controlled so as to precisely follow the natural frequency of the mechanical system of said compressor, including said drive coil, said vibrating compressor is operated in the resonant state.
In conventional refrigerators using this type of vibrating compressor, main switching elements are caused to switch by comparing the (h.sub.FE .times.I.sub.B) product of transistors working as said main switching elements with the collector current I.sub.C thereof, as shown in U.S. Pat. No. 3,916,271 and U.S. Pat. No. 4,027,211, both obtained by the present applicant. That is, switching operation is performed at such a timing that the value of the collector current I.sub.C corresponding to a current flowing in said drive coil becomes larger than the value obtained by multiplying the base current I.sub.B given in synchronism with the vibration of the mechanical system by a coefficient h.sub.FE.
Although the conventional refrigerators can be operated without inconvenience under normal operating conditions, when operated under operating conditions subject to severe temperature changes, the conventional refrigerator may not be operated in the best operating conditions even if the switching point of said transistors is preset to the optimum state because the coefficient h.sub.FE of said transistors varies with temperature. Furthermore, in case said coefficient h.sub.FE is reduced to an extremely low level due to secular variation, said d-c/a-c converter circuit may deviate from said resonant state, causing abnormal oscillation.
In addition, said coefficient h.sub.FE significantly differs with individual transistors. With a standard transistor, there is a difference as large as 5 times between the minimum and maximum values of said coefficient h.sub.FE. For this reason, when a plurality of transistors are used in combination in a push-pull or bridge circuit system, it is necessary to select those transistors having almost the same value of said coefficient h.sub.FE.
On the other hand, said collector current I.sub.C does not necessarily accurately represent the state of compressor, that is, the vibrating positions of piston. For example:
(i) When the d-c voltage fed to the d-c/a-c converter circuit contains ripple components, the wave form of said collector current I.sub.C may be distorted from that obrained when the d-c voltage does not contain ripple components. PA1 (ii) The impedance of the electrical system including the drive coil, which is given by the output impedance as viewed from the d-c/a-c converter circuit to the output side, may change when unwanted inductive components, etc. are superimposed on lead wires for feeding current to the drive coil. This may distort the wave form of said collector current I.sub.C. PA1 (iii) Furthermore, the same may be said of the case where unwanted inductive components are superposed on lead wires for feeding current to the d-c/a-c converter circuit.
For the reasons cited in (i) and (ii) above, starting troubles or unwanted beat noises may occur, and in extreme cases, unstable oscillation of said d-c/a-c converter circuit may result.
Consequently, in the conventional refrigerators of this type, the power source side and output side impedances of said d-c/a-c converter circuit have to be set at a sufficiently low level, and at the same time, a d-c power source having less ripple components has to be used. To overcome these problems, efforts have been made to obtain a vibrating compressor which uses a completely different system from the conventional one as switching conditions of said main switching elements and yet is capable of precisely following the vibration of the mechanical system.